1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for variable minimum input voltage based switching in an electronic power control system to, for example, reduce switching related losses.
2. Description of the Related Art
Power control systems often utilize a switching power converter to convert alternating current (AC) voltages to direct current (DC) voltages or DC-to-DC. Switching power converters provide power factor corrected and regulated output voltages to many devices that utilize a regulated output voltage. Exemplary devices that utilize a regulated output voltage include lamps, such as light emitting diode and gas discharge type lamps, cellular telephones, computing devices, personal digital assistants, and power supplies.
FIG. 1 depicts an electronic power control system 100 that includes a controller 102 that controls the operation of, and, thus, the delivery of power by switching power converter 104. Voltage source 106 supplies an alternating current (AC) input voltage VIN to a full bridge diode rectifier 108. The voltage source 106 is, for example, a public utility, and the AC voltage VIN is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The full bridge rectifier 108 supplies a rectified AC voltage VX to the switching power converter 104. Capacitor 110 filters high frequency components from rectified voltage VX.
To control the operation of switching power converter 104, controller 102 generates the control signal CS0 to control conductivity of field effect transistor (FET) switch 124. The control signal CS0 is a pulse width modulated signal. Control signal CS0 generally has a frequency within a range of 20 kHz to 100 kHz. Each pulse of control signal CS0 turns switch 124 ON (i.e. conducts), and the inductor current iL energizes inductor 126. Diode 128 prevents current flow from link capacitor 130 into switch 124. When the pulse ends, the inductor 126 reverses voltage polarity (commonly referred to as “flyback”) and the inductor current iL charges link capacitor 130 through diode 111. The switching power converter 104 is a boost-type converter because the link voltage VLINK is greater than the rectified input voltage VX. Controller 102 operates the switching power converter 104 to maintain an approximately constant link voltage VLINK for load 132 and provide power factor correction. Load 132 can be any type of load that utilizes the link voltage, such as lamps, such as light emitting diode and gas discharge type lamps, cellular telephones, computing devices, personal digital assistants, and power supplies.
To control operation of switching power converter 104, controller 102 monitors a sense signal VX—SENSE, which represents the rectified voltage VX, and monitors a sense signal VLINK—SENSE, which represents the link voltage VLINK. Controller 102 uses the sense signals VX—SENSE and VLINK—SENSE to generate the control signal CS0 using well-known control circuitry.
FIG. 2 depicts exemplary signal waveforms 200, which are associated with electronic power control system 100. The signal waveforms 200 are the rectified input voltage VX and control signal CS0A. Control signal CS0A represents two exemplary cycles 202 and 204 of the control signal CS0 in FIG. 1. Referring to FIGS. 1 and 2, since each cycle 202 and 204 of input voltage VX is one-half of the original cycle of the input supply voltage VIN, each full cycle 202 and 204 of input voltage VX spans from 0° to 180° of the rectified input voltage VX. In one embodiment, when controlling switching power converter 104, controller 102 continuously generates the control signal CS0A for the full 0°-180° of each cycle, including cycles 202 and 204, of input voltage VX.
FIG. 3 depicts exemplary signal waveforms 300, which are associated with electronic power control system 100. The signal waveforms 300 are the utilized portion VX—USED of the rectified input voltage VX and control signal CS0B. Control signal CS0B represents two exemplary cycles 302 and 304 of the control signal CS0 in FIG. 1. Referring to FIGS. 1 and 3, when controlling switching power converter 104, controller 102 generates the control signal CS0B only when the input voltage VX is greater than a static, minimum line voltage VLINE—MIN—STATIC. In at least one embodiment, generating pulses of control signal CSB only when the line voltage VX is greater than the minimum line voltage VLINE—MIN facilitates a simpler calculation of the duration for each pulse width of control signal CS0B. The minimum line voltage VLINE—MIN—STATIC is, thus, static and is set to facilitate the simpler calculations. The minimum line voltage VLINE—MIN—STATIC corresponds to the first and last 5° of the input voltage VX, i.e. VLINE—MIN—STATIC corresponds to 5° and 175° of VX. Thus, controller 102 does not generate the control signal CSB for the first and last 5° of the input voltage VX.
Referring to FIGS. 2 and 3, generating control signal CS0A continuously allows switching power converter 104 to deliver full power to load 132 and facilitates accurate power factor correction. However, control signal CS0A incurs switching losses throughout the entire cycle of each cycle of input voltage VX. Switching losses are incurred, for example, by charging and discharging parasitic capacitances associated with switch 124. Generating control signal CS0B using the static, minimum line voltage VLINE—MIN—STATIC reduces switching losses of switch 124 by eliminating the switching losses that would otherwise be incurred when the controller 102 stops generating control signal CSB, e.g. for the first and last 5° of the input voltage VX. However, the intermittent generation of control signal CS0B causes harmonic distortion when the current iL is not proportional to the input voltage VX. Because of the abrupt restarts of control signal CS0B during the first and last 5° of each cycle of the input voltage VX, the input voltage VX is not proportional to the current iL during the first and last 5° of each cycle of the input voltage VX. Maximum total harmonic distortion of a load on a public voltage distribution systems, such as an embodiment of voltage source 106, is often regulated, such as by the IEC61000-3-2 specification of the International Engineering Consortium. The IEC61000-3-2 specification specifies limits for harmonic current emissions applicable to electrical and electronic equipment having an input current up to and including 16 A per phase and intended to be connected to public low-voltage distribution systems.”